Three-dimensional quantum well transistor and fabrication method

ABSTRACT

Three dimensional quantum well transistors and fabrication methods are provided. A quantum well layer, a barrier layer, and a gate structure can be sequentially formed on an insulating surface of a fin part. The gate structure can be formed over the barrier layer and across the fin part. The QW layer and the barrier layer can form a hetero-junction of the transistor. A recess can be formed in the fin part on both sides of the gate structure to suspend a sidewall spacer. A source and a drain can be formed by growing an epitaxial material in the recess and the sidewall spacer formed on both sidewalls of the gate electrode can be positioned on surface of the source and the drain.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.201310024090.3, filed on Jan. 22, 2013, the entire contents of which areincorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to the field of semiconductor technologyand, more particularly, relates to three-dimensional quantum well(3D-QW) transistors and fabrication methods.

BACKGROUND

In a typical MOS device, a channel region is formed by doping thesemiconductor substrate with impurities. The channel region may haveboth majority carriers and ionized impurities. The majority carriers maybe scattered by the ionized impurities during a migration process. Themobility of carriers and the device performance can be reduced. Asdimensions of semiconductor devices continue to shrink, short-channeleffect may seriously affect device performance.

High electron mobility transistors (HEMTs) have a channel region formedby a hetero-junction. The hetero-junction is formed by an undopedquantum well (QW) layer and a barrier layer on the undoped QW layer.Two-dimensional electron gas may be formed in the QW layer and may bemigrated in a horizontal plane. The two-dimensional electron gas is usedas carriers of the transistor. Because the QW layer is undoped, thetwo-dimensional electron gas is not affected by the scattering of theionized impurities, which provides high mobility to the carriers. Deviceperformance is effectively improved with reduced short channel effectsand reduced threshold voltage.

For HEMTs, however, the gate may have less or weak control on theextended area of the channel region under the sidewall spacer of thegate, which may result in a non-uniform density of the two-dimensionalelectron gas in the channel region with high resistance in the channelregion and with reduced device performance.

BRIEF SUMMARY OF THE DISCLOSURE

One aspect of present disclosure includes a method of forming atransistor. A buffer layer is formed on the semiconductor substrate. Afin part is formed by etching the buffer layer and the fin part isformed of an insulating material. An insulating layer is formed on thesemiconductor substrate having a top surface lower than a top surface ofthe fin part. A QW layer is formed on surface of the fin part and abarrier layer is formed on surface of the QW layer. A gate structure isformed on the barrier layer across the fin part and on the insulatinglayer. The gate structure includes a gate dielectric layer on each ofthe insulating layer and the barrier layer, and a gate electrode on thegate dielectric layer. A sidewall spacer is formed on both sidewalls ofthe gate structure, followed by forming a recess in the fin part on bothsides of the gate structure to suspend the sidewall spacer. A source anda drain are then formed by growing an epitaxial material in the recess.Each of the source and the drain includes a side edge aligned with asidewall edge of the gate structure such that a channel region in the QWlayer under the gate structure having a same width with the gatestructure and the channel region does not extend to under the sidewallspacer.

Another aspect of present disclosure includes a transistor. Thetransistor includes a fin part and an insulating layer each disposed ona semiconductor substrate, the insulating layer having a top surfacelower than a top surface of the fin part. The transistor includes aquantum well (QW) layer disposed on the fin part and a barrier layerdisposed on the QW layer. A gate structure is disposed on the barrierlayer across the fin part and on the insulating layer. The gatestructure includes a gate dielectric layer on each of the insulatinglayer and the barrier layer, and a gate electrode on the gate dielectriclayer. A source and a drain are disposed on both sides of the gatestructure and within the fin part. Each of the source and the drain hasa side edge aligned with a sidewall edge of the gate structure. Thetransistor further includes a sidewall spacer disposed on both sidewallsof the gate electrode and on surface of each of the source and the drainsuch that a channel region in the QW layer under the gate structurehaving a same width with the gate structure and the channel region doesnot extend to under the sidewall spacer.

Other aspects or embodiments of the present disclosure can be understoodby those skilled in the art in light of the description, the claims, andthe drawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-21 depict an exemplary three-dimensional quantum well (3D-QW)transistor at various stages during its formation in accordance withvarious disclosed embodiments;

FIG. 22 depicts a relationship between a drain current and a gatevoltage for a transistor; and

FIG. 23 depicts an exemplary method for forming a 3D-QW transistor inaccordance with various disclosed embodiments.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of thedisclosure, which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Three-dimensional quantum well (3D-QW) transistors and methods offorming the 3D-QW transistors are provided. On an insulating surface ofa fin part formed from a buffer layer, a QW layer, a barrier layer, anda gate structure can be sequentially formed. The gate structure can beformed on the barrier layer and across the fin part. The QW layer andthe barrier layer can form a hetero-junction of the transistor acrossthe fin part. The disclosed transistors and fabrication methods canreduce resistance of the transistor channel region, improve thesource-drain current, and increase the thermal stability of thetransistor.

FIG. 23 depicts a flowchart of an exemplary method for forming atransistor consistent with the disclosed embodiments, and FIGS. 1-21depict an exemplary transistor corresponding to certain stages duringits formation in accordance with various disclosed embodiments.

Referring to FIG. 1, a semiconductor substrate 100 is provided (StepS11).

The semiconductor substrate 100 can be made of a material including,e.g., silicon, germanium, silicon germanium, silicon carbide, galliumarsenide, and/or other suitable semiconductor materials. Thesemiconductor substrate 100 can include a composite structure such assilicon on insulator (SOI). Suitable materials and structures can beselected for the semiconductor substrate 100, depending on desiredsemiconductor devices (including e.g., the 3D-QW transistor to be formedand/or other possible devices) to be formed on the semiconductorsubstrate 100. The type of the semiconductor substrate is not limitedherein.

In one embodiment, the semiconductor substrate 100 can be a siliconsubstrate. The exemplary silicon substrate can have a crystal plane(111). The crystal plane (111) of the silicon substrate can have alattice constant and/or a lattice structure same or similar to thebuffer layer to be subsequently formed. This can facilitate high qualityepitaxial growth of the buffer layer.

Referring to FIG. 2, a buffer layer 200 is formed on surface of thesemiconductor substrate 100 (Step S12).

In this embodiment, the buffer layer 200 can be made of a materialincluding, e.g., AlN. The buffer layer 200 can have a thickness rangingfrom about 1 micron to about 2 microns. The buffer layer 200 can beformed by a process including, e.g., a metal organic chemical vapordeposition, an atomic layer deposition, a molecular beam epitaxy, etc.

The buffer layer 200 can include an undoped AlN as a desired insulatorto isolate the semiconductor devices from the semiconductor substrate.AlN buffer layer can have high thermal conductivity of about 3.4 W/cmKto more easily introduce heat generated by the semiconductor devicesinto the semiconductor substrate and thus to improve thermal stabilityof the semiconductor devices.

Moreover, the buffer layer 200 can be configured between thesemiconductor substrate 100 and a subsequently-formed QW layer. Thesilicon substrate is an indirect band gap semiconductor, and thesubsequently-formed QW layer can be a direct band gap semiconductor. Thesilicon substrate and the subsequently-formed QW layer can belattice-mismatched. The lattice constant of an AlN material is betweenlattice constants of the silicon substrate and the QW layer. The AlNbuffer layer can provide a lattice basis for a subsequent growth of theQW layer.

In one embodiment, the buffer layer can be formed with higher quality onthe crystal plane (111) of the silicon substrate because the latticeconstant of the silicon substrate is relatively close to the latticeconstant of the AlN buffer layer. The buffer layer with higher qualitycan allow less defects to be formed to a fin part during a subsequentetching of the buffer layer. Deposition quality of the QW layer on thefin part can thus be improved.

Referring to FIG. 3, the buffer layer 200 is etched to form a fin part201 (Step S13). The fin part 201 can be formed by, e.g., a plasmaetching. The fin part 201 can have a height the same as or less than athickness of the buffer layer 200. The fin part 201 can have a heightranging from about 1 micron to about 2 microns.

Referring to FIG. 4, an insulating layer 101 is formed on thesemiconductor substrate 100 (Step S14).

In one embodiment, the insulating layer 101 can be made of a materialincluding, e.g., silicon oxide. The insulating layer 101 can have athickness of about 50 nm to about 500 nm. In one embodiment, a thermaloxidation method can be used to oxidize the exposed surface of thesemiconductor substrate 100 (i.e., not covered by the fin part 201) toform a silicon oxide layer thereon as the insulating layer 101. Theinsulating layer 101 can be used as an isolation structure between asubsequently formed gate structure and the semiconductor substrate. Inother embodiments, a wet oxidation method can be used to form theinsulating layer 101.

Referring to FIG. 5, a QW layer 202 is formed on surface of the fin part201 (Step S15).

The QW layer 202 can be made of a material including group III-V and/orgroup II-VI materials. For example, the QW layer 202 can be made of amaterial including, e.g., GaN, AlGaN, InGaN, Ge, and/or other suitablematerials. The QW layer 202 can have a thickness ranging from about 10nm to about 100 nm.

The QW layer 202 can be formed by a process including, e.g., ametal-organic chemical vapor deposition, an atomic layer deposition, amolecular beam epitaxy, etc. The fin part 201 can be made of a materialincluding, e.g., AlN. The fin part 201 can be made of a single crystalmaterial having a lattice structure, while the QW layer 202 can also bemade of a semiconductor material having a lattice structure. The QWlayer 202 can therefore be selectively formed on the surface of the finpart 201. The QW layer 202 is not formed on the surface of theinsulating layer 101.

Referring to FIG. 6, a barrier layer 203 is formed on surface the QWlayer 202 (Step S16).

The barrier layer 203 can be made of a material including, e.g., AlN,AlGaN and/or AlInN. The barrier layer 203 can have a thickness rangingfrom about 1 nm to about 10 nm. The barrier layer 203 can be formed by aprocess including, e.g., a metal-organic chemical vapor deposition, anatomic layer deposition, a molecular beam epitaxy process, etc. Forexample, the barrier layer 203 can be made of AlInN. The barrier layer203 can be selectively formed on surface of the QW layer 202.

The barrier layer 203 and the QW layer 202 can form a hetero-junctionstructure. The barrier layer 203 can have an energy bandgap greater thanthe QW layer 202 to form a large energy band offset on the junctionsurface and to form strong polarization effect between the barrier layer203 and the QW layer 202. The QW layer can be used as a non-dopednarrow-bandgap channel layer.

A potential well can be formed on surface area of the QW layer 202 nearthe barrier layer 203. Electrons can move from the barrier layer 203into the QW layer 202 but be limited within the potential well at thejunction surface between the QW layer 202 and barrier layer 203.Electrons can only be free to move in a plane parallel to the junctionplane but be limited to move in a vertical direction to the junctionplane, thereby forming a two-dimensional electron gas.

Because the QW layer 202 is undoped, the two-dimensional electron gas inthe potential well is not subject to the scattering of the ionizedimpurities and thus has a high mobility. By controlling the gate voltageof subsequently formed gate structure, the depth of the potential wellof the hetero-junction structure can be controlled, thereby controllingsurface density of the two-dimensional electron gas in the potentialwell and controlling the operating current of the transistor.

Referring to FIG. 7, a capping layer 204 is formed on the surface of thebarrier layer 203.

For example, the cap layer 204 can be made of a material including,e.g., GaN or AlGaN. The capping layer 204 can have a thickness rangingfrom about 1 nm to 3 nm. For example, the cap layer 204 can be made ofGaN. The capping layer 204 can be formed by a process including, e.g., ametal-organic chemical vapor deposition, an atomic layer deposition, ora molecular beam epitaxy, etc. The formed capping layer 204 can reducethe gate leakage current and increase the breakdown characteristics ofthe transistor. In other embodiments, the capping layer 204 may not beformed.

Referring to FIG. 8, a gate dielectric material 205 is formed on surfaceof the cap layer 204 and on the insulating layer 101 (Step S17).

The gate dielectric material 205 can include high-K dielectric materialsincluding, e.g., SiO₂, ZrO₂, Al₂O₃, HfO₂, HfSiO₄, La₂O₃, HfSiON and/orHfAlO₂. The gate dielectric material 205 can have a thickness rangingfrom about 1 nm to 5 nm, for example, from about 1 nm to about 3 nm.

The gate dielectric material 205 can be formed by a process including,e.g., a metal-organic chemical vapor deposition, an atomic layerdeposition, or a molecular beam epitaxy process. In other embodimentswhere the cap layer 204 is not formed, the gate dielectric material 205can be formed directly on surface of the barrier layer 203.

Referring to FIG. 9, a gate metal layer 300 is formed on surface of thegate dielectric material 205 (Step S17).

The gate metal layer 300 can be made of a material including, e.g., NiAuand/or CrAu. The gate metal layer 300 can be formed by, e.g., a chemicalvapor deposition or a sputtering process. In some cases, a chemicalmechanical polishing process can be included to planarize surfaces.

FIG. 10 depicts a cross-sectional view of the transistor in FIG. 9 alonga length (or longitudinal) direction of the fin part 201 after formationof the gate metal layer 300.

Referring to FIG. 11, the gate metal layer 300 and the gate dielectricmaterial 205 are etched to form a gate structure 210. Thus, the gatestructure 210 includes a gate dielectric layer 206 and a metal gateelectrode 207 as shown in FIG. 11.

For example, the gate structure 210 can be formed by first forming apatterned mask layer on the gate metal layer 300, the patterned masklayer covering a position for forming the gate structure 210; and usingthe patterned mask layer as an etch mask to dry etch the gate metallayer 300 and the gate dielectric material 205 to form the metal gateelectrode 207 and the gate dielectric layer 206. The gate dielectriclayer 206 can expose a surface portion of the insulating layer 101. Thegate structure 210 can be formed on the surface of the cap layer 204 andacross the fin part 201.

FIG. 12 depicts a cross-sectional view of the transistor in FIG. 11along a length (or longitudinal) direction of the fin part 201 afterforming the gate structure 210. As shown, after etching to form the gatestructure 210, a portion of the cap layer 204 on both sides of the gatestructure 210 and on the surface of the fin part 201 can be exposed.

FIG. 13 depicts a cross-sectional view of the transistor along a length(or longitudinal) direction of the fin part 201. As shown in FIG. 13, asidewall spacer 208 is formed on both sidewalls of the gate structure210 (Step S18).

The sidewall spacer 208 can be made of a material including, e.g.,silicon nitride. The sidewall spacer 208 can protect the metal gateelectrode 207 and the gate dielectric layer 206 during a subsequentprocess for forming a source/drain.

Referring to FIG. 14, a recess 301 is formed within the fin part 201 atboth sides of the gate structure 210 and the sidewall spacer 208 (StepS19).

For example, the recess 301 can be formed by: using the gate structure210 and the sidewall spacer 208 as a mask to sequentially etch thecapping layer 204, the barrier layer 203, the QW layer 202, and/or aportion of the fin part 201 to form the recess 301. The etching processcan include a dry etching process such as a plasma etching process usingBCl₃ as plasma source.

Referring to FIG. 15, a recess 301′ is formed by removing a portion ofeach of: the cap layer 204, the barrier layer 203, the QW layer 202,and/or the fin part 201, under the sidewall spacer 208 to suspend thesidewall spacer 208 over the fin part 201. For example, the method forsuspending the sidewall spacer 208 can include a wet etching.

In one embodiment, diaminoethane can be used in an etching solution toremove a portion of the barrier layer 203 made of AlInN under thesidewall spacer. NaOH solution having a molar concentration of about 30%to about 50% can be used as an etching solution to remove a portion ofeach of the QW layer 202 and the cap layer 204 that are made of GaNunder the sidewall spacer. HF solution can be used to remove a portionof the fin part 201 made of AlN under the sidewall spacer. The use ofwet etching solution can provide a simple process without assistancefrom photolithography and can selectively remove materials of eachportion under the sidewall spacer 208.

The recess 301′ can be used as a source/drain recess region for formingsource/drain therein. The buffer layer is partially etched.

Referring to FIG. 16, a source 302 and a drain 303 can be formed, e.g.,re-grown, in the recess 301′ (Step S20).

The two-dimensional (2D) electron gas in the QW layer 202 of thetransistor can be used as a carrier. When the source 302 and the drain303 are doped with N-type dopants, an N-type transistor can be formed.For example, the source 302 and the drain 303 can be made of a materialincluding, e.g., N-doped GaN. The source 302 and drain 303 can be madeof a method including a metal organic chemical vapor deposition, anatomic layer deposition, or a molecular beam epitaxy processes.

For example, after a GaN layer is grown in the recess 301′, the GaNlayer can be doped with N-type ions to form an N-type doped GaN layer asthe source 302/drain 303 of the transistor. In other embodiments, anin-situ doping process can be used to dope the N-type dopants to formthe source 302 and the drain 303, whiling forming the GaN layer.

In one embodiment, because the fin part 201 can be made of a materialincluding, e.g., AlN, which is difficult to be doped, a recess can beformed first in the fin part 201 at both sides of the gate structure 210and then an epitaxial semiconductor material that is easily to be dopedcan be grown within the recess to form the source and the drain. As asemiconductor material, GaN can be easily doped. In addition, becauseAlN has fixed or regular lattice structure, selective epitaxy can beused to grow the GaN layer in the recess and to dope the GaN layer toform the source 302 and the drain 303.

The sidewall spacer 208 can be positioned having a bottom surface on thesource 302 and on the drain 303 such that the QW layer 202 and barrierlayer 203 can form a hetero-junction region having a width to be used asa channel region of the transistor. Such width of the hetero-junctionregion (or the channel region) can be the same as a width of the gatestructure 210. When a gate voltage is applied to the gate structure 210,by adjusting the gate voltage, the density of the two-dimensionalelectron gas in the channel region under the gate structure 210 can becontrolled, thereby controlling the work current of the transistor.

The channel region under the gate structure 210 can be affected by thegate voltage. When the gate voltage is greater than a threshold voltage,the two-dimensional electron gas of the channel region can have auniform density to provide the channel region with a low resistance.

In a case when the disclosed wet etching process is not used to suspendthe sidewall spacer 208 on both sides of the gate structure, ahetero-junction structure can be formed under the non-suspended sidewallspacer, which can be used as an extended region of the channel region.Because no biased voltage can be applied to the non-suspended sidewallspacer, the extended region of the channel region under thenon-suspended sidewall spacer can have a two-dimensional electron gaswith a density less than the density of the two-dimensional electron gasin the channel region 210 exactly beneath the gate structure.

As such, the extended region of the channel region exactly under thenon-suspended sidewall spacer can have a great resistance and thus theentire channel region (including the extended region) can have a reducedaverage density of the electron gas. This reduces the source-draincurrent of the transistor. In this manner, the removal of the extendedregion of the channel region under the sidewall spacer can reduceresistance of the channel region and increase the source-drain currentof the transistor. No underlap or overlap can be generated between thegate and the source/drain, due to the removal of the extended region ofthe channel region under the sidewall spacer.

Referring to FIG. 17, a metal electrode 304 can be formed on the source302 and the drain 303.

The metal electrode 304 can be made of a material including, e.g., NiAuand/or CrAu. The metal electrode 304 can be formed by, e.g., asputtering process or an evaporation process. The metal electrode 304may reduce a contact resistance of the source 302 and the drain 303.

Referring to FIG. 18, an interlayer dielectric layer 400 is formed onthe metal gate electrode 207 and the insulating layer 101. FIG. 18 showsa cross sectional view of the transistor along a direction perpendicularto a length of the fin part.

The interlayer dielectric layer 400 can be made of a material including,e.g., an insulating dielectric material such as silicon oxide,phosphorus-doped silicon oxide, boron-doped silicon oxide, and/or boronphosphorus-doped silicon oxide.

FIG. 19 depicts a cross-sectional view of the transistor along adirection perpendicular to a length of the fin part 201, after formingthe interlayer dielectric layer 400. As shown, the inter-layerdielectric layer 400 can cover the sidewall spacer 208 and the metalelectrode 304 on each of the source 302 and the drain 303.

For example, the interlayer dielectric layer 400 can be formed by aprocess including, e.g., a chemical vapor deposition. After depositingan interlayer dielectric material on surface of the metal gate electrode207, the insulating layer 101, the sidewall spacer 208, and the metalelectrode 304, a chemical mechanical polishing process can be performedto planarize the entire surface and to form the interlayer dielectriclayer 400.

Subsequently, a metal interconnect structure can be formed on surface ofthe interlayer dielectric layer 400 to connect the semiconductor device(not shown) with the metal interconnect structure.

FIGS. 20-21 depict various exemplary transistors each including multiple3D-QW transistors. Note that although two 3D-QW transistors areillustrated in FIGS. 20-21, one of ordinary skill in the art wouldappreciate that any desired number of 3D-QW transistors can be formed onthe substrate 100 as shown in FIGS. 20-21.

For example, FIG. 20 depicts an exemplary transistor including two 3D-QWtransistors simultaneously formed on adjacent different fin parts andover the semiconductor substrate 100. The cross-sectional view of theexemplary transistor in FIG. 20 is along a direction perpendicular to alength of the fin parts.

As shown in FIG. 20, two fin parts 201 can be simultaneously formed froma buffer layer on a semiconductor substrate. According to the methoddescribed above in FIGS. 1-18, a 3D-QW transistor can be formed on eachof the two fin parts. During the formation, when etching a gate metallayer 300 and a gate dielectric material 205 to form a gate structure210 (e.g., as shown in FIG. 9), an etching process can be performed toetch materials of the gate metal layer and the gate dielectric materialbetween the two transistors to expose an insulating layer 101 to isolatethe two transistor gate structures. An interlayer dielectric layertransistor 400 can then be formed on the transistor and on theinsulating layer to cover surfaces thereof. Because all of theinterlayer dielectric layer 400, the insulating layer 101, and the finparts 201 are formed of insulating material(s), no additional isolationstructures (e.g., shallow trench isolation structure) are needed forelectrical isolation between the two transistors.

FIG. 21 depicts an exemplary transistor including two 3D-QW transistorssimultaneously formed on a same fin part and over the semiconductorsubstrate 100. The cross-sectional view of the exemplary transistor inFIG. 21 is along a length direction of the fin part.

By using the above-described method as depicted in FIGS. 1-18, two ormore 3D-QW transistors can be simultaneously formed on the fin part 201.Adjacent two 3D-QW transistors can be spaced apart. After the formationof adjacent 3D-QW transistors, a portion of each of the QW layer, thebarrier layer, and/or the capping layer between the two transistors onsurface of the fin part 201 can be removed to form an interlayerdielectric layer 400 on the fin part between the adjacent transistors.The two transistors formed on the same fin part are connected at thebottom by the fin part 201 that is formed by insulating materials. Aportion of the fin part can also further isolate the two 3D-QWtransistors. No additional isolation structures are therefore needed.

In conventional methods for forming transistors, additional isolationstructures have to be formed, e.g., using silicon oxide as the isolationdielectric material. Silicon oxide has a thermal conductivity of about0.014 W/cmK. Such low thermal conductivity can affect heat transfer fromthe transistors over the substrate to the substrate. As disclosed, 3D-QWtransistor can be formed on the fin part 201. The gate structure can beformed having a sufficient contact area with the fin part to effectivelyimprove an area of the channel region of the transistor and to improve agate length of the gate structure. Transistor performance can thus beimproved and heat dissipation efficiency of the transistor can beimproved. In addition, insulting materials such as AlN can be used asthe fin part. The exemplary AlN material can have a thermal conductivityof about 3.4 W/cmK, which has two orders of magnitude higher than theconventionally used silicon oxide. The use of the fin part as anisolation medium can facilitate heat transfer from the transistorsconfigured on the semiconductor substrate to the semiconductor substrateto improve stability of the 3D-QW transistor.

An exemplary 3D-QW transistor is also provided herein. For example,referring back to FIG. 18, a 3D-QW transistor can include asemiconductor substrate 100; an insulating fin part 201 and aninsulating layer 101 formed on surface of the semiconductor substrate100, the insulating layer 101 having a top surface lower than the topsurface of the fin part 201; a QW layer 202 formed on surface of the finpart 201; a barrier layer 203 formed on the QW layer 202; and gatestructure 210 formed on the barrier layer 203 and across the fin part,the gate structure 210 including a gate dielectric layer 206 over thebarrier layer and a gate electrode 207 on the gate dielectric layer. The3D-QW transistor can further include an interlayer dielectric layer 400.

In one embodiment, the semiconductor substrate 100 can include a siliconsubstrate having a crystal plane (111). The fin part 201 can be made ofa material of AlN having a height ranging from about 1 micron to about 2microns. The insulating layer can have a thickness ranging from about 50nm to about 500 nm and can be made of a material of silicon oxide.

The QW layer 202 can have a thickness ranging from about 10 nm to about100 nm and can be made of a material from group III-V and/or groupII-VI. The QW layer 202 can be made of GaN, AlGaN, InGaN, and/or Ge.

The barrier layer 203 can have a thickness ranging from about 1 nm toabout 10 nm, and can be made of AlN, AlGaN, and/or AlInN. The fin part201 can be made of a material of AlN and can have a height ranging fromabout 1 micron to about 2 microns. The insulating layer can have athickness ranging from about 50 nanometers to 500 nanometers and can bemade of a material of silicon oxide.

In one embodiment, a capping layer 204 can be disposed between the gatestructure 210 and the barrier layer 203. The capping layer 204 can havea thickness ranging from about 1 nm to about 3 nm. The capping layer 204can be made of a material of GaN. In other embodiments, the disclosedtransistor can be formed without the capping layer 204.

The gate dielectric layer 206 can have a thickness ranging from about 1nm to about 5 nm and can be made of a material including, e.g., SiO₂,ZrO₂, Al₂O₃, Hf0 ₂, HfSiO₄, La₂O₃, HfSiON and/or HfAlO₂.

Referring back to FIG. 19, another exemplary 3D-QW transistor can beformed further including: a source 302 and a drain 303 formed on the finpart at both sides of the gate structure 210; a sidewall spacer 208located on both sidewalls of the gate structure 210 and on surface ofthe source 302 and the drain 303; and a metal electrode 304 on eachsurface of the source 302 and drain 303.

For example, the source 302 and the drain 303 can be N-type dopedincluding, e.g., N-doped GaN. The metal electrode 304 on surface of thesource 302 and drain 303 can be made of a material including, e.g., NiAuand/or CrAu. The metal electrode can reduce a contact resistance of thesource and the drain.

As disclosed, the 3D WQ transistor can have a channel region between asource 302 and a drain 303. The channel region can have a width the sameas the gate structure 210. No extended region from the channel regionunder the sidewall spacer 208, which can effectively increase electrondensity in the channel region with reduced resistance of the channelregion and with improved source-drain current.

As disclosed, 3D-QW transistors can be formed on the fin part 201. Thegate structure can be formed having a sufficient contact area with thefin part to effectively improve an area of the channel region of thetransistor and to improve a gate length of the gate structure.Transistor performance can thus be improved and heat dissipationefficiency of the transistor can be improved. In addition, insultingmaterials such as AlN can be used as the fin part. The exemplary AlNmaterial can have a thermal conductivity of about 3.4 W/cmK, which hastwo orders of magnitude higher than the conventionally used siliconoxide. The use of the fin part as an isolation medium can facilitateheat transfer from the transistors configured on the semiconductorsubstrate to the semiconductor substrate to improve stability of the3D-QW transistor.

FIG. 22 depicts a relationship between a drain current and a gatevoltage of a transistor having different underlap gate lengths.

In this example, a width of the fixed gate structure 210 L_(g) is about18 nm, the extended region of the channel region located under thesidewall spacer 208 can have a width L_(un) of about 0 nm, 2 nm, 4 nm, 6nm, 8 nm, and 10 nm respectively. As indicted by FIG. 22, under a samegate voltage, as the width L_(un) of the extended region of the channelregion located under the sidewall spacer 208 decreases, the draincurrent increases.

Therefore, removal of extended region of the channel region locatedunder the sidewall spacer 208 on both sides of the gate structure 210can allow a same width for the channel region and the gate structure ofthe transistor to significantly reduce resistance of the channel regionof the 3D-QW transistor and to improve the source-drain current.

In a particular embodiment, a non-planar III/V quantum well device withAlN for isolation can be formed on a silicon substrate. The AlN withenhanced thermal dissipation compared with GaN, InGaN or AlGaN can beused for buffer layer and thus for the fin part. The source and drainregions of the transistor can be formed by etching and re-growing100-nm-GaN regions with silicon doping to provide a high n-typeconduction. Exemplary AlInN barrier/GaN quantum well/AlN bufferstructure can be etched to form the device source and drain recessregions and leave AlN buffer layer partially etched. Doped GaN can bere-grown on the source and drain recess regions to form thesource/drain. No STI structures are needed for device isolation.

The embodiments disclosed herein are exemplary only. Other applications,advantages, alternations, modifications, or equivalents to the disclosedembodiments are obvious to those skilled in the art and are intended tobe encompassed within the scope of the present disclosure.

What is claimed is:
 1. A method for forming a transistor comprising:providing a semiconductor substrate; forming a buffer layer on thesemiconductor substrate; forming a fin part by etching the buffer layer,wherein the fin part is formed of an insulating material; forming aninsulating layer on the semiconductor substrate, the insulating layerhaving a top surface lower than a top surface of the fin part; forming aquantum well (QW) layer on surface of the fin part; forming a barrierlayer on surface of the QW layer; forming a gate structure on thebarrier layer across the fin part and on the insulating layer, the gatestructure including a gate dielectric layer on each of the insulatinglayer and the barrier layer, and a gate electrode on the gate dielectriclayer; forming a sidewall spacer on both sidewalls of the gatestructure; forming a recess in the fin part on both sides of the gatestructure to suspend the sidewall spacer; and forming a source and adrain by growing an epitaxial material in the recess, each of the sourceand the drain including a side edge aligned with a sidewall edge of thegate structure such that a channel region in the QW layer under the gatestructure having a same width with the gate structure and the channelregion does not extend to under the sidewall spacer.
 2. The methodaccording to claim 1, where forming the recess to suspend the sidewallspacer includes: using the gate structure and the sidewall spacer as amask to dry etch the barrier layer, the QW layer, and the fin part; andusing a wet etching process to remove a portion of each of the barrierlayer, the QW layer, and the fin part, under the sidewall spacer tosuspend the sidewall spacer.
 3. The method according to claim 1, whereinthe buffer layer is formed on a crystal plane (111) of the semiconductorsubstrate.
 4. The method according to claim 1, wherein the buffer layeris made of a material including AlN, and the buffer layer has athickness ranging from about 1 micron to 2 microns.
 5. The methodaccording to claim 1, wherein the QW layer is made of a materialincluding a compound selected from group III-V and group II-VI.
 6. Themethod according to claim 1, wherein the QW layer is made of a materialincluding GaN, AlGaN, InGaN, or Ge, and the QW layer has a thicknessranging from about 10 nm to 100 nm.
 7. The method according to claim 1,wherein the barrier layer is made of a material including a compoundselected from group III-V and group II-VI.
 8. The method according toclaim 1, wherein the barrier layer is made of a material including AlN,AlGaN, or AlInN, and the barrier layer has a thickness ranging fromabout 1 nm to 10 nm.
 9. The method according to claim 1, wherein, priorto forming the gate structure, a capping layer is formed on the barrierlayer, and wherein the capping layer has a thickness ranging from about1 nm to 3 nm and is made of a material including GaN.
 10. The methodaccording to claim 1, wherein each of the source and the drain is madeof a material including N-doped GaN.
 11. The method according to claim1, further including forming a metal electrode on each of the source andthe drain, wherein the metal electrode is made of a material includingNiAu or CrAu.
 12. A transistor comprising: a semiconductor substrate; afin part and an insulating layer each disposed on the semiconductorsubstrate, the insulating layer having a top surface lower than a topsurface of the fin part; a quantum well (QW) layer disposed on the finpart; a barrier layer disposed on the QW layer; a gate structuredisposed on the barrier layer across the fin part and on the insulatinglayer, the gate structure including a gate dielectric layer on each ofthe insulating layer and the barrier layer, and a gate electrode on thegate dielectric layer; a source and a drain disposed on both sides ofthe gate structure and within the fin part, each of the source and thedrain having a side edge aligned with a sidewall edge of the gatestructure; and a sidewall spacer disposed on both sidewalls of the gateelectrode and on surface of each of the source and the drain such that achannel region in the QW layer under the gate structure having a samewidth with the gate structure and the channel region does not extend tounder the sidewall spacer.
 13. The transistor according to claim 12,wherein the buffer layer is disposed on a crystal plane (111) of thesemiconductor substrate.
 14. The transistor according to claim 12,wherein the fin part is made of a material including AlN, and the finpart has a height ranging from about 1 micron to about 2 microns. 15.The transistor according to claim 12, wherein the QW layer is made of amaterial including a compound selected from group III-V and group II-VI.16. The transistor according to claim 12, wherein the QW layer is madeof a material including GaN, AlGaN, InGaN, or Ge, and the QW layer has athickness ranging from about 10 nm to 100 nm.
 17. The transistoraccording to claim 12, wherein the barrier layer is made of a materialincluding AlN, AlGaN, or AlInN, and the barrier layer has a thicknessranging from about 1 nm to 10 nm.
 18. The transistor according to claim12, wherein each of the source and the drain is made of a materialincluding N-doped GaN.
 19. The transistor according to claim 12, whereina metal electrode is formed on each of the source and the drain, whereinthe metal electrode is made of a material including NiAu or CrAu. 20.The transistor according to claim 12, wherein the fin part formed of theisolating material is used as an isolation structure and has a thermalconductivity sufficiently high to provide sufficient heat transfer fromthe transistor to the semiconductor substrate.